During the past year, a retrospective epidemiological study of Beta-hemolytic, group D, streptococcal infections was initiated. Conjugative hemolysin plasmids were isolated from five clinical Streptococcus faecalis isolates of diverse geographical origins. Cloned DNA, containing the hemolysin gene(s) from one of these plasmids (pAD1), hybridized to the same two Eco RI restriction fragments from each of the five plasmids. Hyberidization experimens using total plasmid DNA suggested that the plasmids shared between 40 percent and 100 percent homology with each other. Two of the plasmids, pAD1 from strain DS16 and pAMGramma1 from strain DS5, were 100 percent homologous and had identical Eco RI restriction patterns (eight fragments each). Two of the S. faecalis strains, DS16 and JH1, shared a sufficient number of "atypical" properties to suggest a common ancestral origin: 1) their hemolysin plasmids, pAD1 (DS16) and pJH2 (JH1), were at least 90 percent homologous; 2) each strain contained a chromosome-borne tetracycline resistance transposon with at least partial homology between them; 3) each strain harbored an antibiotic resistance plasmid, pAD2 (DS16) and pJH1), mediatinig resistance to streptomycin, kanamycin, and erythromycin, the latter trait being transposable in each case.